This invention relates to an optical scanning system and more particularly, to a system utilizing a lens array titled at an angle with respect to an object line which projects an image of the object line onto a tiered array of linear image sensors.
Large arrays of solid-state photosensors, e.g. charge coupled imaging arrays, are currently used in applications such as video cameras and document scanners. The semiconductor fabrication techniques currently employed to manufacture these arrays limit the maximum physical dimension to approximately one inch. These are further limitations, both physical and electrical, which establish a minimum center-to-center distance between adjacent photosensor elements. Thus, in a single linear array, there exists a maximum number of photosensor elements which can be practically fabricated. Since the number of image points resolved with such an array is proportional to the number of photosensor elements in the array, there are potential scanning applications where the available number of photosensors in one array is insufficient to produce the desired image resolution. To overcome this problem, it has been proposed that several short, linear photosensor arrays be aligned along a single axis to provide the effect of one array long enough to provide for all of the bit detection needed. Implementation of long photosensor arrays from a plurality of shorter photosensor arrays requires that a crossover from one array to the next occur at some position along the scan line. Aligning the arrays, for example, so that the last detector of the previous array and the first detector of the next array lie on common centers, requires sensitive opto-mechanical tolerances which could be costly, difficult to maintain in a machine environment, and inconvenient to implement on a production basis.
An improved optical scanning system which addresses this problem is disclosed in U.S. Pat. No. 4,080,633 assigned to the same assignee as the present invention. This reference discloses a flying spot scanning system which utilizes a lens array tilted at an angle to a document scan line to convey the bit position information content of the document scan line onto a tiered array of light sensitive detectors disposed along an axis perpendicular to the axial direction of the scan line. The lens array has a lens of proper focal length associated with each scan segment of the document scan line and the detector array has a detector unit associated with each scan segment of the document scan line, with each detector unit having a detector element corresponding to each bit position of the scan segment associated with that detector unit, such that the information content of each scan segment is recorded on a different one of the tiered detector units of the detector array via a different lens of the lens array. Due to a twisting or axial reorientation of the information content of the scan line produced by the tilted lens array, a tiered or two dimensional (area) array of detector units can be utilized to record the information content of the scan line. Unlike long linear detector arrays, area detector arrays with large numbers of detector elements more easily can be manufactured by conventional manufacturing techniques since they only require a rectangular-shaped semiconductor material wafer on the order of one inch by one quarter inch.
The type of geometry disclosed in the above referenced patent has several characteristics which may not be appropriate for certain applications. The focusing requirements dictate the use of small lenses with short center-to-center spacings. The lens f numbers are relatively high and, to maintain radiometric efficiency, scanning speeds must be at relatively slower rates.
It is an object of the invention to provide an improved optical scanning system utilizing a tilted lens array to image scan segments onto an area array of detector units.